if you measure the voltage and you see anything under 12 do NOT try to crank the engine.. put a 4 -6 amp battery charge on there and let it sit till done which make take 3-5 hrs. My truck sat for about 4 weeks with no trickle charge so when i checked the voltage it was 12.2.. I got out my

Schumacher 15 Amp Battery Charger using the 4 amp setting and let it run.. The display shows 55% when it starts and goes to 100 when done.

I had the same issue, my truck sits for weeks to months on end especially during winter so I after driving I put on the trickle charge you can get at harbor freight for like 5 bucks.. https://www.harborfreight.com/automa...ger-69955.html you can get a catalog and find them for 6 bucks.

put it on, plug it in and forget till spring or you get a chance to take the truck out..

If you try to crank at low voltage you will, not maybe, but will, damage the cells in the battery shorting the life of it.

 

I actually started it twice with the battery low, and now it's got a battery tender tucked in a nice spot bolted to the passenger side battery... Which I presume will charge BOTH?

 

if they both conneced it would

 

Double check that. On GMC trucks, if you have the dual batteries on the GAS motor, they are isolated from each other. But if you have the Diesel motor, they are wired together without any intervening circuitry from the factory. I know back in the day my dads ford F250 had two batteries that were isolated from each other for running a slide-in camper. Kind of like how motorhomes have "chassis" and "house" batteries.

Worth checking how your particular truck is wired up. Easy enough to do, connect the charger to one, and check the voltages of each.

 

After testing the engine block heater I did a careful examination of the list of things covered by the incredibly expensive Extended Warranty. Then I called the Ford ESP people, and after a few transfers through people telling me "No, I don't see anything that SAYS the engine block heater is covered", I got to a person in the right department (or cube or just by dumb luck) who looked at whatever she looked at and said "Yes, PROVISIONALLY, the engine block heater IS covered." "Um, 'Provisionally'?" "Yes, that means if the dealer says it's bad, it's covered." And she gave me a reference number just in case there was a problem with the deailer.
Waited a week for the local Ford dealer to "have time" to see if I was correct and the engine block heater was dead. Got a call a few hours later that it was indeed not working. And that "we don't have the part, but it'll be here tomorrow, and we'll have it fixed tomorrow."
Day 2, no word from dealer
Day 3, no word from dealer
End of Day 3, I called the dealer to ask what was going on. Nobody at the service desk knew. After a surprisingly long wait on hold, the guy came back and said "We don't have the part, but it's scheduled to be here tomorrow." Which was exactly what they told me on DAY 1.
Late-ish Day 4, finally got a call that the truck was fixed...

So, though I'm not impressed with their speed OR their communications, hopefully it actually is fixed.

Thanks for the help testing the block heater.

 

Wow, it's stuff like that which gives dealers a bad name. Hope it works great for you from now on.